1. Field of the Invention
This invention pertains to hydraulically actuated clutches and brakes of an automatic transmission. More particularly the invention pertains to compression spring assemblies that return a hydraulic piston to its disengaged position.
2. Prior Art
An hydraulically actuated clutch or brake used in automatic transmissions to hold and release elements of a planetary gearset in order to produce the various gear ratios of the transmission, includes an hydraulic cylinder, a piston mounted for movement within the hydraulic cylinder into contact with two sets of clutch friction discs, a pressure plate that reacts force applied by the piston to the friction discs, and a spring assembly that produces a force tending to move the piston away from the friction discs and toward an opposite end of the cylinder--to the disengaged position.
The spring assembly includes a large number of compression springs angularly spaced about the longidutinal axis of the transmission and clutch. One end of each spring is held against a fixed spring retainer against which the spring force is applied, and a retainer located at the opposite end of the compression springs, the retainer bearing against an adjacent surface of an hydraulic piston. A conventional assembly of this type has the hydraulic cylinder formed with studs, angularly spaced about the longitudinal axis and sized to receive a compression spring on each stud. The stud is provided to locate the compression springs correctly within the transmission assembly and to support the spring so that when compression force is developed between the piston and retainer, the spring stays in a stable configuration with its coils aligned mutually and with the axis of the stud. The stud prevents transverse displacement of the coils of the compression springs.
FIG. 1 of U.S. Pat. No. 4,759,234 illustrates a configuration of the spring pack supported with a stud integrally formed with an hydraulic piston.
It is important in the design of automatic transmissions that its lateral dimensions be minimized in order to fit within the relatively small space provided for the transmission in a front-wheel drive vehicle. This objective is realized by reducing as much as possible the radial extent from the longitudinal axis of the transmission of each component for maintaining the distance of the components.
In the design of hydraulically actuated friction clutches and brakes, multiple compression springs (usually about 36 springs) are used to return the piston to its disengaged position within the hydraulic clutch. In order to accommodate the space requirements of the transmission, it is preferable that the return springs have a relatively high slenderness ratio, i.e., that the length of the springs be relatively large in comparison to the diameter. However, it is well-known that a compression spring having a high slenderness ratio is susceptible to compression instability displacement, a condition in which the spring distorts from a linear line of travel such that the intermediate portion of the spring is displaced angularly with respect to the coils of the spring located near the ends of the springs.
In clutch assemblies of the type used in automatic transmission clutches and brakes, a large number of compression springs is used in a single assembly to return a piston to its disengaged position. When compression instability occurs in such an assembly of springs, the entire spring pack rotates angularly at one end with respect to the position at the opposite axial end of the spring pack.
The effect of compression instability deformation is to produce a variable spring constant, i.e., the relationship of the spring force to its displacement from the unloaded or free length of the spring.
Instability deformation of the compression spring pack produces a variable rate of clutch engagement because the return spring offers a variable and unpredictable force in opposition to the hydraulic pressure force developed on the piston that causes engagement of the hydraulic clutch and brake.
Instability deformation of the compression return springs also causes excessive clutch wear because the friction plates of the clutch are not fully engaged when the clutch is actuated hydraulically because of excessive resistance opposing that action produced by the clutch pack.